In the past, polyalkylene glycol-based hydraulic fluids have generally required the presence of water therein in order to provide a degree of fire resistance sufficient to meet the Factory Mutual Research, Group I, Class No. 6930 approval (so-called "FM approval") with regard to the flame resistance of the fluid. Water is undesirable as an additive in hydraulic fluids for several reasons, most notably due to operating pressure limitations imparted by the vapor pressure of water and potential corrosion problems caused by water on the metal surfaces of the hydraulic system.
As an alternative to the use of water additives in polyalkylene glycol-based hydraulic fluids, polyol ester-type fluids, e.g. trioleate esters of trimethylol propane, typically utilize high molecular weight polymer anti-mist additives in order to provide FM approval. Unfortunately, such anti-mist additives tend to degrade when subjected to the shear forces typically encountered by hydraulic fluids during use. Accordingly, hydraulic fluids containing such prior art anti-mist additives tend to have relatively short useful lives of a few months or less, dependant upon the operating conditions and service requirement for the particular application for which the hydraulic fluid is employed. Additionally, polyol ester type lubricant bases are also subject to hydrolysis under certain conditions, which can alter the performance characteristics of the hydraulic fluid.
In view of the above, new non-aqueous hydraulic fluids that exhibit an improved combination of flame retardancy and shear stability would be highly desired by the hydraulic fluid manufacturing community.